Crop coefficients and water balance in an irrigated cornfield during the 2004 growing period (Apr.27 to Oct. 12) in the upper Yellow River basin in Inner Mongolia, China, were analyzed using the dual crop coefficient approach recommended by the FAO, in which crop transpiration and soil evaporation were evaluated based on the crop coefficients and the reference evapotranspiration determined from meteorological conditions. During the initial stage when ground cover by corn plants was approximately less than 10%, the mean crop coefficient was 0.50 and soil evaporation accounted for 84% of evapotranspiration. In the crop development, mid-season and late season stages when most of the ground surface was covered with corn plants, the mean crop coefficients were 0.63, 1.18 and 0.99, respectively, and soil evaporation accounted for only 10-20% of evapotranspiration. During the entire growing period, cumulative crop transpiration and soil evaporation were 389.1 mm and 126.0 mm, respectively, and corresponded to 75.5% and 24.5% of the cumulative evapotranspiration estimated by this approach (515.1 mm), which almost equated with the measurement made by the Bowen ratio method (512.3 mm). Cumulative precipitation was 294.6 mm and about 150 mm of water was irrigated at the end of the crop development stage. These results suggest that approximately 30% of the rainwater and the irrigation water was lost by soil evaporation and about 70 mm of water was supplied to the root zone by capillary rise from the shallow water table (-1 m to -2 m).
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